The present invention relates to a fixing apparatus used in copiers, printers, facsimile machines or the like to thermo-compression-bond unfixed toner on a sheet and fix it onto the sheet.
In late years, a technique as shown in FIG. 9 has been developed to a fixing apparatus for electrophotographic systems. In this technique, a belt type fixing apparatus comprises a fixing roller R1, a heating/tension roller (hereinafter referred to as xe2x80x9cheating rollerxe2x80x9d) R3, a fixing belt B wound around both the fixing and heating rollers, and a pressing roller R2 for pressing the fixing roller R1 upward through the fixing belt B. Further, the belt type fixing apparatus is combined with a preheating device of a recording medium D. According to this fixing apparatus, a nip region can be set at a lower temperature by virtue of the preheating of the recording medium D. This allows the heat capacity of the fixing belt B to be reduced. Thus, when passing through the nip region, the fixing belt B is quickly cooled down to provide accelerated aggregation of toner to be released from the fixing belt B at the outlet of the nip region, and consequently enhanced releasability between the fixing belt B and the toner, so that a clear fixed toner image will be obtained without undesirable offset even if no oil or a small amount of oil is applied. This fixing apparatus is known as an effective solution of technical problems in releasability and oil application which have not been solved by a heating roller system.
The structure of the above conventional belt type fixing apparatus will be briefly described bellow. The fixing apparatus comprises the fixing roller R1, the pressing roller R2 disposed below the fixing roller R1, the heating roller R3 disposed on the side of the fixing roller R1 (on the upstream side along the feeding direction of the recording medium), and the fixing belt B wound around both the fixing roller R1 and the heating roller R3.
An oil coating roller R4 is provided above an upper belt run of the fixing belt B. Further, a guide plate G serving as a support member for the recording medium is provided below an lower belt run of the fixing belt B with leaving a certain distance therebetween to define a heating passage P for heating the recording medium between the lower belt run of the fixing belt B and the upper surface of the guide plate B. A pressing lever U is adapted to press the heating roller R3 in the direction causing the heating roller R3 to get away from the fixing roller R1 so as to provide a desired tension of the fixing belt B, and the heating roller R3 is driven by the fixing roller R1, so that the fixing belt B can be stably rotated without slipping or sagging.
The belt type fixing apparatus has various features as described above. On the other hand, the belt type fixing apparatus exposes extremely deteriorated releasability as a sheet is large-sized, particularly when used in an electrophotographic machine compatible with A-3 size sheets (or A4 cross-feed), and it transpires that a desired releasability is hardly secured without a separation pawl. Thus, there is a need for improving such a disadvantage.
In view of the above circumstances, it is therefore an object of the present invention to provide a fixing apparatus capable of reliably releasing a sheet having fixed toner thereon from a fixing belt without any separation pawl even if the sheet is large-sized.
It is another object of the invention to provide a fixing apparatus capable of reliably releasing a sheet having fixed toner thereon from a fixing belt even in an electrophotographic machine compatible with A-3 size sheets.
In order to achieve the above objects, according to a first aspect of the present invention, there is provided a fixing apparatus comprising: a fixing roller; a pressing roller in rotational contact with the fixing roller; a biasing member for applying a biasing force to the pressing roller so as to bring the pressing roller into press contact with the fixing roller at a given pressure; a heating roller disposed apart from the fixing roller; a fixing belt wound around both the heating and fixing rollers in an endless manner; and a heat-generating device embedded in the heating roller to heat the fixing belt so as to heat unfixed toner on a sheet passing through a rotational contact region between the fixing and pressing rollers, whereby when the sheet having unfixed toner on a surface thereof is passed through the rotational contact region along one direction, the unfixed toner is fixed onto the sheet. In this fixing apparatus, an interior angle xcex8 between an axis X and an axis Y is arranged to satisfy the following relationship,
xe2x88x9230xc2x0 less than xcex8 less than xe2x88x9280 xc2x0
where X is an axis connecting the center of the fixing roller and the center of the pressing roller, and Y is an axis along a biasing direction in which the pressing roller applies the biasing force to the fixing roller in the rotational contact region, wherein on the basis of the axis X, a positive sign is provided to the angle xcex8 when the axis Y is located on a sheet-feed side where the sheet is fed into the rotational contact region, and a negative sign is provided to the angle xcex8 when the axis Y is located on a sheet-discharge side where the sheet is discharged from the rotational contact region.
According to a second aspect of the present invention, there is provided a fixing apparatus comprising: a fixing roller; a pressing roller in rotational contact with the fixing roller; a biasing member for applying a biasing force to the pressing roller so as to bring the pressing roller into press contact with the fixing roller at a given pressure; a heating roller disposed apart from the fixing roller; a fixing belt wound around both the heating and fixing rollers in an endless manner; and a heat-generating device embedded in the heating roller to heat the fixing belt so as to heat unfixed toner on a sheet passing through a rotational contact region between the fixing and pressing rollers, whereby when the sheet having unfixed toner on a surface thereof is passed through the rotational contact region along one direction, the unfixed toner is fixed onto the sheet. In this fixing apparatus, the pressing roller is rotatably pivoted to a swingable member swingably supported on a given pivot shaft, the biasing member being coupled with the swingable member to allow the pressing roller to be brought into press contact with the fixing roller, and an interior angle xcex8 between an axis X and an axis Y being arranged to satisfy the following relationship,
xe2x88x9230xc2x0 less than xcex8 less than xe2x88x9280xc2x0
where X is an axis connecting the center of the fixing roller and the center of the pressing roller, and Y is an axis along a direction perpendicular to a straight line which passes through the rotational contact region and connects the pivot shaft and the rotational contact region, wherein on the basis of the axis X, a positive sign is provided to the angle xcex8 when the axis Y is located on a sheet-feed side where the sheet is fed into the rotational contact region, and a negative sign is provided to the angle xcex8 when the axis Y is located on a sheet-discharge side where the sheet is discharged from the rotational contact region.
According to a third aspect of the present invention, there is provided a fixing apparatus comprising: a fixing roller; a pressing roller in rotational contact with the fixing roller; and a biasing member for applying a biasing force to the pressing roller so as to bring the pressing roller into press contact with the fixing roller at a given pressure, whereby when a sheet having unfixed toner on a surface thereof is passed through a rotational contact region between the fixing and pressing rollers along one direction, the unfixed toner is fixed onto the sheet. In this fixing apparatus, the fixing roller is disposed to be opposed to the surface of the sheet having unfixed toner, the pressing roller being disposed on the opposite side of the pressing roller to allow the sheet to be nipped therebetween, and the rotational contact region having an inlet region for receiving the sheet therein and an outlet region for discharging the sheet therefrom, wherein the pressing roller is arranged to provide a pressure distribution in the rotational contact region along the feeding direction of the sheet in which a pressure at the outlet region is higher than that at the inlet region.
In the fixing apparatus according to the first or second aspect of the present invention, the angle xcex8 may be arranged to satisfy the following relationship.
xe2x88x9240xc2x0 less than xcex8 less than xe2x88x9255xc2x0
In the fixing apparatus according to the first or second aspect of the present invention, the heating roller may be disposed on the opposite side of the pressing roller on the basis of a feeding path of the sheet interposed therebetween. Further, the fixing roller and the pressing roller may be disposed vertically apart from one another, and the heating roller may be disposed above the fixing roller.
In the fixing apparatus according to the first or second aspect of the present invention, the pressing roller may be disposed on the opposite side of the fixing roller on the basis of a feeding path of the sheet interposed therebetween. Further, the pressing roller may be disposed with a displacement in the discharging direction of the sheet from a position symmetrically opposite to the fixing roller on the basis of the feeding path.
In the fixing apparatus according to the second aspect of the present invention, the pivot shaft may be disposed with a displacement in the discharging direction of the sheet greater than that of the center of the pressing roller.
In the fixing apparatus according to the first, second or third aspect of the present invention, the fixing belt includes an endless-shaped substrate made of metal. Further, the endless-shaped substrate may be made of electroformed nickel.
Alternatively, the fixing belt may include an endless-shaped substrate made of synthetic resin. Further, the endless-shaped substrate may be made of polyimide.